“Alarming” levels of sugar and calories are hidden in milkshakes sold in restaurants and fast food chains, with some containing more than six times the daily recomended sugar intake for a child.

Family restaurant Toby Carvery is ranked as the worst offender with its “Unicorn Freakshake” containing 39 teaspoons of sugar – despite experts recommending that the average seven to ten year old should not consume more than 6g of sugar a day, according to a report from Action on Sugar. The next most extreme example is Five Guys Banana and Chocolate Shake with an excessive 37 teaspoons of sugar – the equivalent of drinking more than four cans of cola.

The group, writing in Sugar Awareness Week, is calling for mandatory traffic light nutrition labelling across all menus and a ban on the sale of milkshakes that exceed a calorie limit of 300 kcal per serving. Registered nutritionist Kawther Hashem, a Action on Sugar researcher based at Queen Mary University of London, said: “Some of these milkshakes contribute to excess sugar and calorie intake, and it is shocking this information is hidden from the consumer. It is time the government introduced legislation to force companies to be more transparent about what is in their products by displaying clear information online and in outlets.”

In supermarket-bought drinks, Muller’s Milk Frijj Chocolate Flavour was the higherst in sugar with 42g of sugar in a 400ml bottle.

Dr Linda Greenwall, founder of the Dental Wellness Trust said: “These findings are remarkable, especially given tooth decay among children in Britain is now at a record high, largely because food and drink products are packed with unnecessary sugar.

“The out of home sector and parents must take immediate responsibility by significantly reducing the amount of sugar given to children.”

Earlier this year, the Scottish Government said it would urge the UK government to push for mandatory “front of pack” labelling that would help consumers easily identify healthier and unhealthier foods – and also to include milk-based drinks in the Soft Drinks Industry Levy, which currently provides an exemption for milk-based drinks containing more than 75 per cent milk.

A Toby Carvery spokesman said: “Freakshakes only feature on our main menu and are not targeted at children.”